Subject: I-D Action:draft-ietf-tsvwg-byte-pkt-congest-02.txt

From: [email-masked]
Date: Mon, 12 Jul 2010 13:15:02 -0700 (PDT)

A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the Transport Area Working Group Working Group of the IETF.

	Title           : Byte and Packet Congestion Notification
	Author(s)       : B. Briscoe, J. Manner
	Filename        : draft-ietf-tsvwg-byte-pkt-congest-02.txt
	Pages           : 39
	Date            : 2010-07-12

This memo concerns dropping or marking packets using active queue
management (AQM) such as random early detection (RED) or pre-
congestion notification (PCN).  We give two strong recommendations:
(1) packet size should not be taken into account when transports read
congestion indications, not when network equipment writes them, and
(2) byte-mode packet drop variant of AQM algorithms, such as RED,
should not be used to drop fewer small packets.

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http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-byte-pkt-congest-02.txt

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From: Michael Menth <[email-masked]>
Date: Tue, 13 Jul 2010 11:18:03 +0200

Hi Bob,

from my perspective, PCN is not an AQM scheme. It's just a meter and marker.

I just browsed abstract and intro of the document and found it a bit 
confusing. I think you mean that meters (or equivalent entities) should 
take packet sizes into account, but the decision whether to mark/drop a 
packet should be independent of its size. Then, I fully agree, although 
at first read I understood the text in a different way.

"And if the probability of dropping a packet depends on its byte-size it 
is called byte-mode drop, whereas if the drop probability is independent 
of a packet's byte-size it is called packet-mode drop."

Why not just say packet-size-(in)dependent (PSI,PSD) dropping or 
marking? I find this wording more intuitive and less confusing.

Best wishes,

    Michael

-- 
PD Dr. habil. Michael Menth, Assistant Professor
University of Wuerzburg, Institute of Computer Science
Am Hubland, D-97074 Wuerzburg, Germany, room B206
phone: (+49)-931/31-86644 (new), fax: (+49)-931/31-86632 (new)
mailto:[email-masked]
http://www3.informatik.uni-wuerzburg.de/research/ngn

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From: Bob Briscoe <[email-masked]>
Date: Tue, 13 Jul 2010 13:28:17 +0100

Michael,

>from my perspective, PCN is not an AQM scheme. It's just a meter and marker.

An AQM typically involves a meter and a marker/dropper. There is 
surely no way PCN is anything but It must be AQM.

The wiki page on AQM defines it as "a technique that consists in 
dropping or ECN-marking packets before a router's queue is full"
<http://en.wikipedia.org/wiki/Active_Queue_Management>
PCN certainly satisfies that description.

>confusing. I think you mean that meters (or equivalent entities) 
>should take packet sizes into account, but the decision whether to 
>mark/drop a packet should be independent of its size. Then, I fully 
>agree, although at first read I understood the text in a different way.

OK, I'll take a look - I thought we'd made it clear.

>"And if the probability of dropping a packet depends on its 
>byte-size it is called byte-mode drop, whereas if the drop 
>probability is independent of a packet's byte-size it is called 
>packet-mode drop."
>

They are terms from the RED implementation, not my invented terms. 
Yes, they are confusing, but I can't rewind history...

Bob

>>implementation to automatically retrieve the ASCII version of the

________________________________________________________________
Bob Briscoe,                                BT Innovate & Design

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From: Michael Menth <[email-masked]>
Date: Tue, 13 Jul 2010 16:46:19 +0200

Hi Bob,

> PCN certainly satisfies that description.
I believe that the common property of the mechanisms in the focus of 
this draft is that they meter traffic in some way and perform some 
actions on the base of the metering result. Whether this is for the 
purpose of actively managing a (physical/virtual) queue is probably not 
so important. That's why I would avoid AQM in this context. But that's a 
not so important detail anyway.

>> "And if the probability of dropping a packet depends on its byte-size 
>> it is called byte-mode drop, whereas if the drop probability is 
>> independent of a packet's byte-size it is called packet-mode drop."
>>
You need to extend the definition of "packet/byte-mode drop" to marking 
algorithms anyway. The ambition of this document is to describe more 
than RED and so it's justified to use a more general notation that also 
satisfies applications that do not drop packets. It's confusing to talk 
about a packe/byte-mode drop for PCN which does not drop packets at all. 
The packet(byte)-mode drop of RED is just an example of a packet-size 
(in)dependent action.

Regards,

    Michael